Polyurethane coating materials typically comprise a catalyst, and in this context not only acidic compounds but also, in particular, tertiary amines and/or metallic compounds, such as various tin compounds, for example, more particularly dibutyltin dilaurate and dibutyltin oxide, are employed.
The employment of tin-containing catalysts is to be avoided in coating materials, as elsewhere, because of the toxicity inherent in many tin compounds. The EU Commission's Working Group on Classification and Labelling” have categorized dibutyltin oxide (DBTO) and dibutyltin dilaurate (DBTL) accordingly.
WO04/029121 describes polyurethane compositions which are stabilized in terms of the reactivity of the composition by addition of acids with a pKa range between 2.8 and 4.5, these acids being able to be utilized at the same time as catalyst. Acids used in this context and with a pKa range between 2.8 and 4.5 include, for example, benzoic acid, hydroxybenzoic acid, salicylic acid, phthalic acid, and so on. The compositions preferably comprise no further catalyst, although in addition it is also possible to use the typical known polyurethane catalysts, such as tertiary amines or amidines or organometallic compounds, such as tin compounds more particularly. Where amines are used as catalyst, it is necessary to employ great care in the selection of the type of amine and its amount, since the aminic catalysts are able in part to eliminate the stabilizing action of the organic acids added.
Further, EP-A-1 273 640 describes 2-component coating materials comprising a polyol component and a crosslinker component, consisting of aliphatic and/or cycloaliphatic polyisocyanates or the polyisocyanates derived from them by polymerization, allophanatization, biuretization or urethaneization, where 0.1 to 95 mol % of the originally free isocyanate groups present have reacted with bisalkoxysilylamine. These coating materials can be used for OEM finishing and, following their complete curing, exhibit good scratch resistance in conjunction with high resistance towards environmental influences. Nevertheless, these coating materials have a particularly strong propensity towards ongoing crosslinking, which means that the scratch resistance of the coatings—directly after thermal curing to completion—is no more than inadequate. In addition, the severe ongoing crosslinking has adverse consequences for the weathering stability, since the risk of stress cracks is increased.
Furthermore, WO 2001/98393, WO08/74491, WO 08/74490, WO 08/74489, WO09/077181 and WO 10/149236 disclose coating material compositions which comprise at least one hydroxyl-containing compound (A), at least one compound (B) containing isocyanate groups and silane groups, and a catalyst suitable for the crosslinking of the silane groups. Catalysts used are phosphorus-containing catalysts, more particularly phosphorus- and nitrogen-containing catalysts. These coating material compositions have the advantage over conventional polyurethane coating materials of a significantly enhanced scratch resistance in conjunction with good weathering stability. In need of improvement, however, is the curing of the coating material compositions at low cure temperatures, of the kind generally employed for automotive refinishing and/or for the coating of plastics substrates and/or utility vehicles.
WO09/077180, moreover, describes coating material compositions of the aforementioned kind that comprise the at least one hydroxyl-containing compound (A), at least one compound (B) comprising isocyanate groups and silane groups, and, as catalyst (D), at least one phosphoric acid compound blocked with an amine, the amine used comprising more particularly bicyclic amines, preferably diazabicyclooctane; this does improve the curing at low cure temperatures, in relation to the coating materials of WO 2001/98393, WO08/74491, WO 08/74490, WO 08/74489, WO09/077181 and WO 10/149236, but even faster curing under the conditions of refinishing and the finishing of utility vehicles is desirable, and so the coatings after curing at 60° C. for just 30 minutes should exhibit a level of curing such that initial assembly operations or demasking can be performed without damage to the coating.
Finally, PCT patent applications PCT/EP2012/052284, PCT/EP2012/051444, PCT/EP2012/051574, PCT/EP2012/054546, not yet laid open, and European patent application No. 12152406.0, not yet laid open, describe coating material compositions which as well as at least one polyhydroxyl group-containing compound (A), at least one polyisocyanate group-containing compound (B) and at least one catalyst (D) based on a zinc-amidine complex, also comprise at least one monomeric aromatic, optionally substituted carboxylic acid (S), whose carboxyl group is conjugated with a π-electron system.